History

Mosquito bites

Patients may not provide a mosquito bite history. The incubation period of West Nile encephalitis (WNE) is 1-6 days and is perhaps as long as 14 days in immunosuppressed patients.

Symptoms

West Nile virus (WNV) infection may result in one of three clinical pictures.

Most infections result in asymptomatic seroconversion or a mild flulike illness.

Alternatively, the infection can result in West Nile fever, which is self-limited and resembles other viral syndromes, including dengue fever. Fever, headache, myalgias and back pain occur. Less-common features include eye pain, gastrointestinal symptoms, and, very rarely, generalized lymphadenopathy. Rash has been described in 20%-50% of cases.
[13]

Lastly, 1 in 150 infections result in neuroinvasive disease/encephalitis. The clinical features of encephalitis are often indistinguishable from other forms of viral encephalitis. The severity of the encephalitis varies, with potential manifestations including flaccid paralysis, extrapyramidal findings, coarse tremor, myoclonus, and substantial cognitive difficulties. Other neurological findings may include demyelinating polyneuropathy and brachial plexopathy.

Less-common features of West Nile virus infection include rhabdomyolysis, myocarditis, orchitis and hepatitis.

The typical incubation period is from 2-14 days.

WNE often begins with nonspecific symptoms, including sore throat, backache, myalgias, and arthralgias. Most patients with WNE present with features of encephalitis and, less commonly, with aseptic meningitis (meningoencephalitis), or both. Symptoms include acute febrile illness accompanied by headache, mental confusion, tremors, or flaccid paralysis. Symptoms are most prominent in very young and elderly patients.

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Physical Examination

The extent and severity of central nervous system (CNS) viral invasion determine the clinical expression of WNE. Most patients are febrile.

Patients who present with encephalitis have mental confusion or disorientation and may have decreased consciousness. Patients with severe cases of West Nile encephalitis (WNE) may present with stupor or coma.

Multifocal chorioretinitis is the most common ophthalmologic manifestation of WNE.
[14] In patients with aseptic meningitis or meningoencephalitis, CNS findings include a stiff neck with meningeal signs, including positive Kernig or Brudzinski signs. Some cases of WNE are accompanied by mild, nonexudative pharyngitis. Generalized adenopathy and an enlarged submental node are rare (1%).
[15]

A study found that approximately 20% of patients in the New York City area who were infected with West Nile virus during a 1999 outbreak presented with truncal maculopapular rash, but this is uncommon.
[15]

The Culex mosquito, common in the eastern United States, is the primary vector responsible for infecting humans with West Nile virus. Prevention of West Nile virus is primarily directed at reducing the mosquito population from May to October and by taking precautions to limit human exposure during these months of high mosquito activity. Image courtesy of the Centers for Disease Control and Prevention.

The geographic distribution of the Japanese encephalitis servocomplex of the family Flaviridae, 2000. Image courtesy of the Centers for Disease Control and Prevention.

States reporting laboratory-positive West Nile virus infection in birds, mosquitoes, animals, or humans between January 1 and August 28, 2002. Image courtesy of the Centers for Disease Control and Prevention.